Because the car has the 'premium' sound package there is an external amplifier behind your glove box. That amp has to be bypassed so your new deck won't amplify the signal and then send it to the factory amp to be amplified again. This causes a very bad sound if you don't bypass the factory amp. The extra part you need is a wiring harness that plugs in the car and routes the power from your new deck directly the harness that runs out to the speakers instead of through the factory amp...hence the name, bypass. HTH.
On a side note, if you have a local Circuit City or Ultimate Electronics or some other reputable stereo shop I would recommend going to them for your stereo install. Most places will not charge you the labor for installing the new deck if you purchase it from them. FWIW.
